1. Field of the Invention
The present invention relates to an overheat protection circuit which is provided in a current path between a power supply and a load, for example, and is used to interrupt a current to the load by interrupting the current path when the load overheats.
2. Description of the Related Art
In DC—DC converters used as power supplies in communication base stations, for example, when the power consumption increases above the requirements, a semiconductor switching element such as an FET, power transistor, etc., which is used as a circuit element, generates excessive heat, the semiconductor switching element finally exhibits thermal runaway, and there are cases in which the semiconductor switching element does not function as a power supply.
Therefore, in an overheat protection circuit for preventing such overheating, a temperature detection element for detecting an overheating condition is required. As the temperature detection element, a temperature fuse, for example, is known. In the case of the temperature fuse, once the circuit is interrupted, the circuit does not return to its original state if left as it is, and accordingly, a new temperature fuse must be provided, which means that associated labor time and cost are needed.
On the other hand, a positive-temperature-coefficient thermistor can be used as a temperature detection element. In the case of the positive-temperature-coefficient thermistor, since the positive-temperature-coefficient thermistor can be returned to its original state after it has cut off, unlike the temperature fuse, there is no need for labor time for providing parts, and accordingly, there is an advantage in that the cost is reduced by a certain amount.
Up to now, as an overheat protection circuit using a positive-temperature-coefficient thermistor, the circuit shown in FIG. 8 has been known. This related overheat protection circuit is composed of a first series circuit having a first resistor R1, a single positive-temperature-coefficient thermistor PTC, and a second resistor R2 and a second series circuit having a load LOAD, a switching transistor Tr1, and a third resistor R3. The positive-temperature-coefficient thermistor PTC is disposed in the vicinity of the switching transistor Tr1 and changes so as to have a resistance value according to the temperature of the switching transistor Tr1. In this case, the temperature detection target by the positive-temperature-coefficient thermistor PTC is the switching transistor Tr1. When the temperature of the switching transistor Tr1 is the Curie point of the positive-temperature-coefficient thermistor PTC or lower, the switching transistor Tr1 is brought into conduction by the positive-temperature-coefficient thermistor PTC having a low resistance value and a current flows to the load LOAD. On the other hand, when the temperature of the switching transistor Tr1 increases due to variations of the current to the load LOAD, etc., the temperature of the positive-temperature-coefficient thermistor PTC reaches the Curie point and exceeds the Curie point, since the resistance of the positive-temperature-coefficient thermistor PTC becomes high, that is, a predetermined value or higher, the base current to the switching transistor Tr1 is reduced by the positive-temperature-coefficient thermistor PTC having a high resistance value, and accordingly, the switching transistor Tr1 is interrupted and the current to the load LOAD is stopped. The detected temperature of the positive-temperature-coefficient thermistor PTC at which the interrupt operation is performed corresponds to an interrupt temperature. In this circuit, since the current to the load stops, the abnormal electrical conduction stops, and accordingly, the temperature of the temperature detection target (switching transistor Tr1, in this case) falls. When the resistance of the positive-temperature-coefficient thermistor PTC becomes lower than a predetermined value as the temperature decreases, since the base current to the switching transistor Tr1 increases beyond a predetermined value, the switching transistor Tr1 returns to the state in which a current flows to the load. The detected temperature of the positive-temperature-coefficient thermistor PTC at which the return operation is performed corresponds to a return temperature. In this circuit, the interrupt temperature is the same as or close to the return temperature.
However, when the temperature of the temperature detection target fluctuates in the vicinity of the interrupt temperature where the resistance of the positive-temperature-coefficient thermistor greatly changes, since the interrupt temperature is the same as or close to the return temperature, there is a risk that the semiconductor switching element alternates between a conductive state and an interrupt state. From the viewpoint of the life of the power supply, etc., it is not desirable that such a conductive state and an interrupt state alternate in a short period of time. Then, if the interrupt temperature and the return temperature can be set such that the interrupt temperature is a predetermined temperature difference higher than the return temperature, alternating between the interrupt operation and the return operation in a short period of time can be avoided. However, in the related overheat protection circuit shown in FIG. 8, such a temperature difference could not be realized.